1. Field of the Invention
This invention relates to the structure of an overhead projector for projecting an image of a sheet material on a screen remote from the overhead projector.
2. Description of Related Art
Conventionally, overhead projectors are often used on conferences, lectures and educational or presentation meetings to project images of, for instance, documentary information and/or drawings recorded on transparent sheet materials, such as chart films, on a remote screen standing vertically and located remote from the overhead projector. Such an overhead projector is configured by various elements, such as a light source, a stage on which a sheet material is placed, a projection lens system, a reflective mirror, etc.
In order to provide an easy understanding of the configuration of the transmission type of conventional overhead projector, reference is made to FIGS. 5 and 6. The transmission type of overhead projector includes its essential optical elements such as a light source 2, a projection lens system 3 and a reflective mirror 4. The light source 2 is fixedly installed beneath a flat transparent stage 1, on which a transparent sheet, such as a film sheet, is placed, so as to illuminate the transparent sheet. The projection lens system 3, which is installed above the stage 1 such that its optical axis X is in alignment with a normal line Y of the flat transparent stage 1, forms and directs an optical image of documents and/or drawings recorded on the transparent sheet to the reflective mirror 4, placed above the projection lens system 3 at an angle of 45 degrees relative to the optical axis X of the projection lens system 3, so as to project an enlarged image of the documents and/or drawings of the transparent sheet onto a remote screen 5 standing approximately vertically.
With such a conventional overhead projector, the reflective mirror 4 is absolutely indispensable in order to reflect and direct the optical axis X of the projection lens system 3 upward and toward the remote screen 5. Specifically, when the optical axis of X of the projection lens system 3 must be directed toward the remote screen 5 at an angle of elevation .theta. relative to a horizontal line H, the reflective mirror 4 is further tilted or inclined half the angle of elevation .theta. from its original position, as indicated by a reference 4A in FIG. 5, wherein it is at an angle of 45 degrees relative to the horizontal line H. In such a manner, the enlarged image of the documents and/or drawings is projected overhead on the remote screen 5 so as to be observed or watched by viewers without any obstruction.
However, with the conventional overhead projector, tilting or inclining of the reflective mirror 4 upward from its original position, in which the reflective mirror 4 is at an angle of 45 degrees, due to a demand of projecting of an image at an angle of elevation so as to form it on the remote screen 5 at the best height for viewers, subjects the image unavoidably to unacceptable distortion. Specifically, as shown in FIG. 6 by example in which the optical axis X of the projection lens system 3 X is unfolded straight for an easy understanding, when there is a relative inclination of an angle of .theta. between the optical axis X of the projection lens system and the normal line Y of the screen 5, which is produced as an angle of elevation by the reflective mirror 4, a rectangular transparent sheet 6 is projected on the screen 5 as an enlarged distorted image 7 having an expanded upper side and a reduced lower side or a reversed trapezoidal configuration. In this instance, the enlarged image 7 is out of focus or indistinctly clear, in particular, at the upper and lower side margins on the screen 5 and consequently, gives viewers even an unpleasant feeling.